Mechanically-actuated “jetting” dispensers are commonly used in the electronics industry, to selectively dispense minute amounts or droplets of a highly viscous fluid material in a non-contact manner onto a circuit board. A typical jetting system includes an applicator or jetting dispenser with a pneumatically-operated needle having a valve element at one end that is configured to selectively engage a valve seat surrounding a discharge passage. Contact between the valve element and valve seat seals off the discharge passage from a chamber supplied with pressurized fluid material. To dispense droplets of the fluid material, the valve element is retracted from contact with the valve seat. A small amount of the fluid material is permitted to flow past the valve seat and into the discharge passage. The valve element is then moved rapidly toward the valve seat to close the space. The rapid reduction in volume extrudes the fluid material through the discharge passage and a shock wave traveling through the fluid and/or the momentum imparted by the extrusion process causes a droplet of the fluid material to be ejected, or “jetted,” from an outlet of the discharge passage. The droplet, which contains a small discrete volume of the fluid material, travels with a ballistic trajectory and eventually lands at a specified location on the circuit board.
Jetting dispensers are able to “fly” above the circuit board at a fixed height and “jet” the material onto an intended application area in a non-contact manner. By rapidly jetting the material “on the fly” (i.e., while the jetting dispenser is in motion), the dispensed droplets may be joined to form a continuous line. Consequently, jetting dispensers may be easily programmed to dispense desired patterns of fluid material. This versatility has made jetting dispensers suitable for a wide variety of applications in the electronics industry.
The jetting dispenser for applying such fluid materials is typically moved by a robot in a pattern across a surface of the circuit board that bears the components. However, the motion is not continuous. When the motion relative to the circuit board places the jetting dispenser at an appropriate dispensing location, the fluid material is dispensed from the jetting dispenser onto the corresponding area or component of the circuit board. The robot then moves the jetting dispenser to the next area or component destined to receive fluid material and, subsequently, dispenses another amount of fluid material.
Each time that fluid material is dispensed, the robot must accelerate the jetting dispenser to move between areas or components on the circuit board. Similarly, the robot must decelerate when slowing the motion of the jetting dispenser in advance of dispensing an amount of fluid material. For example, underfill materials are commonly dispensed in successive continuous events to define short line segments. In that specific application, the stop and start between consecutive line segments lengthens the dispensing time and, furthermore, causes vibration in the jetting dispenser.
Therefore, improved methods of applying fluid materials are needed that are capable of more accurately dispensing amounts of fluid material from a continuously moving jetting dispenser onto a circuit board.